1. Field of the Invention
The present invention relates to a method for manufacturing a stator of a rotor, and more particularly to a method for manufacturing comb teeth of a stator of a small size stepping motor having an outer cylinder diameter of 10 to 100 mm.
2. Related Background Art
As shown in FIGS. 1 and 2, a stepping motor generally comprises a rotor 1 and stators 3.sub.1, 3.sub.2 and 4.sub.1, 4.sub.2, and the stators 3.sub.1, 3.sub.2 and 4.sub.1, 4.sub.2 have a plurality of pole teeth (comb teeth) 3a, 3b, 4a and 4b for mounting exciting coils 5a and 5b circumferentially formed thereon at a constant pitch. The comb teeth 3a, 3b, 4a and 4b are preferably longer in order to attain a large effective magnetic flux created by the excitation the excitation coils 5a and 5b. However, where the comb teeth are manufactured by simply blanking a center of a single frame plate as is done in the prior art, the length of the comb teeth is less than one half of a blanking diameter and there is a limit in attaining a large effective magnetic flux. A method for manufacturing a stator to solve the above problem has been proposed in Japanese Patent Publication No. 57-211964 (Patent Application No. 56-98007).
In the proposed method for manufacturing the stator, as shown in a sectional view of FIG. 11A, a center area 14 of a single frame plate 10 is convex-formed, then as shown in a sectional view of FIG. 11B, the center area 14 is blanked while leaving a comb teeth area 16, and finally as shown in sectional view of FIG. 11C, the comb teeth area 16 is bent normally to the frame plate by burring to manufacture the stator. In accordance with the proposed manufacturing method, the length of the comb teeth may be made larger than one half of the blanking diameter. Numeral 12 denotes an outer periphery of the frame plate, numeral 18 denotes a center hole formed by the blanking, and numeral 20 denotes a blanked piece.
However, the prior art manufacturing method has the following problems:
(1) The stator is usually manufactured by sequentially feeding the frame plate by transfer press. As shown in FIG. 12, in the blanking process, the frame plate 10 is supported by a die 30 from the bottom and it is blanked by a punch 32. Because of the strength of the die, it is not possible to blank the comb teeth of the stator in one step and it is usually done in two or three steps. Numeral 34 denotes a stripper and T.sub.D denotes an interval of holes of the die. For example, when the blanking is made in three steps, as shown in a partial plan view of FIG. 13, areas No. 1 (opposite sides of areas in which two comb teeth 16 are to be formed) are first blanked, then area No. 2 (areas between the areas in which the two comb teeth are Go be formed) are blanked, and finally an area No. 3 18 (center hole) is blanked to form the comb teeth. (FIGS. 14A, 14B and 14C show the three steps for the entire frame plate.) By dividing the blanking into three steps, the interval T.sub.D of the holes 30a of the die can be widened and a problem of the strength of the die is solved.
However, as shown in FIG. 13, when the blanking is made in three steps, the comb teeth 16 are subjected to a force in the second step toward the areas blanked in the first step (direction X) so that they are blanked while they are deformed circumferentially as shown by a broken line. (FIG. 15 shows an entire frame plate). When such deformed comb teeth 16 are bent normally to the frame plate in the subsequent burring process, the comb teeth 16 are bent while they are circumferentially deformed as shown in FIG. 16. When a stepping motor is formed by using such a stator, the indexing of the step angle is not uniform and the positioning accuracy is lowered.
(2) As shown in FIG. 17, when the blanking process is conducted in one step, the problem encountered in the three-step blanking process (the deformation of the comb teeth) is solved but a new problem as described below is encountered. As shown in FIG. 18, the interval T.sub.D of the holes 30a of the die is narrow in a certain area (an area corresponding to ends of the comb teeth), and when the interval T.sub.D is narrower than a thickness t of the frame plate, the strength of the die is not sufficiently large and the die may be readily broken. Thus, mass production is not attained.
As shown in FIG. 19, a blanking clearance (C) is usually set to approximately 0.05 t where t is a thickness of the frame plate. Accordingly, as shown in FIGS. 19 and 18, the blanked piece 20 is frictionally held by the hole 32a of the die and it is stacked. In the one-step blanking process, a holding pressure for the stacked blanked piece acts on the die side wall in the next blanking of the frame plate, and this causes the die to break.
(3) A surface processed steel plate is used as a material for the frame plate. However, the surface processed steel plate includes a variation in the thickness by a rolling property. For example, there is usually a variation of approximately 4% between the thickness in the width direction and in the rolling direction (see FIG. 20). When the comb teeth formed by blanking the frame plate having the variation in the thickness are bent by the burring process, a right angle precision or perpendicular accuracy includes a variation of approximately .+-.1.degree. as shown in FIG. 21 and a product precision of .+-.10.degree. cannot be met. Thus, when the stepping motor is manufactured by using such a frame plate, the performance of the motor is deteriorated.